This invention relates to the preparation of rigid polyurethane foams. This invention further relates to the preparation of rigid polyurethane foams using certain diorganotin compounds in combination with a tertiary amine as gel catalysts for the purpose of delaying initiation of the polymerization reaction. This delay is advantageous in many enduse applications.
Rigid polyurethane foams can be prepared using a variety of well known methods. In some instances it is desirable to prepare the foam at the location where it will be employed. Rigid polyurethane foams may be applied to the outside surface of a storage tank to provide insulation, to the roof of a building or between the inner and outer walls of a building. The urethane foam can be applied by spray. The sprayed-in-place polyurethane foam is economically attractive because of the ease of application by spray. For these applications it is most preferred to employ a two-component system, one of which is a polyfunctional isocyanate such as polymethylene polyphenyl isocyanate. The second component contains the polyol, gel catalyst and blowing agent together with any modifiers or additives. A surfactant such as a siloxane polymer is usually included to achieve a uniform cell structure in the final foam. The second component is often employed as a pre-packaged mixture that is prepared weeks or even months before it is reacted with the isocyanate.
Organotin compounds are typically very active catalysts for the reaction of isocyanates with polyols. Often the reaction is so rapid that the liquid formulation employed to prepare the foam becomes too viscous to flow into every part of the mold or other container into which it is poured. When incomplete filling of the mold occurs, the shape of the final foam does not coincide with that of the mold or other container in which it is formed, and the foam article must therefore often be rejected. The value of rigid urethane foam as an insulating material is considerably reduced if the foam does not completely fill the space between the inner and outer walls of a building or of a container employed to maintain the temperature of a solid or liquid that is stored therein by inhibiting heat transfer.
An objective of this invention is to delay the cream time of rigid polyurethane foams prepared using the combination of a tetravalent organotin compound and a tertiary amine as the gel catalyst. Surprisingly it has now been found that this objective can be achieved using certain sulfur-containing diorganotin compounds as one component of the gel catalyst.